Commercial sulfite pulping has been practiced since 1874. The focus of sulfite pulping is the preservation of cellulose. In an effort to do that, industrial variants of sulfite pulping take 6-10 hours to dissolve hemicelluloses and lignin producing a low yield of fermentable sugars.
Stronger acidic cooking conditions that hydrolyze the hemicelluloses to produce a high yield of fermentable sugars also hydrolyze the cellulose and therefore the cellulose is not preserved.
Sulfite pulping produces spent cooking liquor termed sulfite liquor. Fermentation of sulfite liquor to hemicellulosic ethanol has been practiced primarily to reduce the environmental impact of the discharges from sulfite mills since 1909. Published design data from one of the two known remaining sulfite mills that produces ethanol, shows ethanol yields not to exceed 33% of original hemicelluloses. Ethanol yield is low due to the incomplete hydrolysis of the hemicelluloses to fermentable sugars and further compounded by sulfite pulping side products, such as furfural, methanol, acetic acid and others, inhibiting fermentation to ethanol.
Energy use for ethanol production in said sulfite mill applications is higher than the energy value of the ethanol produced. Furthermore, this sulfite process uses calcium sulfite or ammonium sulfite and has no chemical recovery, therefore chemical losses are high. Because of poor ethanol yield, lower cost of synthetic ethanol production, and the production of ethanol from corn today, only two sulfite mills are known to have continued the practice of hemicellulosic ethanol production to date.
In the 20th century, Kraft pulping eclipsed sulfite pulping as the dominant chemical pulping method.
Kraft pulping however does not hydrolyze the hemicelluloses into fermentable sugars; instead hemicelluloses are in solution with soluble inorganic cooking chemicals and cannot readily be separated.
The number of sulfite pulp mills remaining in operation continues to reduce each year. The main reasons are that when compared to Kraft pulping, sulfite pulping produces inferior strength pulp, requires more cooking time, requires aged wood as the raw material (green wood cannot be readily used), is not feasible on as many different wood species, and lacks an efficient method of chemical recovery therefore chemical losses are high.
Other processes using solvent cooking chemicals have been tried as an alternative to Kraft or sulfite pulping. Original solvent processes are described in the U.S. Pat. No. 1,856,567 to Kleinert et al. and U.S. Pat. No. 2,060,068 to Groombridge et al. Although three demonstration size facilities for ethanol-water (ALCELL), alkaline sulfite with anthraquinone and methanol (ASAM), and ethanol-water-sodium hydroxide (Organocell) were operated briefly in the 1990's, today there are no full scale solvent pulping mills. None of these solvent processes provided for fermentable sugar production from hemicelluloses.
Groombridge shows that an aqueous solvent with sulfur dioxide is a potent delignifying system to produce cellulose from lignocellulosic material.
Furthermore, U.S. Pat. No. 5,879,463 to Proenca reveals that simultaneous delignification and rapid hydrolysis of the entire cellulosic material, both the cellulose and the hemicelluloses, is possible in the presence of an organic solvent and a dilute inorganic acid; however this process does not preserve the cellulose.
Therefore in the prior art of processing lignocellulosic material for the primary purpose of producing cellulose:
a) The sulfite processes to date (including base sulfite and ethanol sulfite) in an effort to preserve the cellulose, do not yield complete hydrolysis of hemicelluloses and produce fermentation inhibitors, thereby resulting in low yields of fermentable sugars in the sulfite liquor and furthermore, low yield of any downstream fermentation products from said sugars.b) Strong acid processing of lignocellulosic material degrades and hydrolyzes both hemicelluloses and cellulose, therefore cellulose is not preserved.c) The Kraft process does not hydrolyze hemicelluloses to fermentable sugars.d) Organic solvent pulping methods did not hydrolyze hemicelluloses to fermentable sugars, i.e., only part were hydrolyzed, not all.e) Treatment of lignocellulosic material with dilute inorganic acid in organic solvent hydrolyzes both cellulose and hemicelluloses and therefore does not preserve the cellulose.f) In the Kraft process lignin condensation causes loss of lignin reactivity.